Biological fouling of circulating water is a known and well documented problem. Several factors contribute to the problem and govern its extent: water temperature; water pH; organic and inorganic nutrients either from air drawn into the system or from materials naturally occurring in the water or continuously supplied during plant operation; aerobic/anaerobic conditions; the presence/absence of sunlight, etc.
Algae, fungi, bacteria, as well as other simple life forms are found in circulating water. The types of microorganisms and the extent of microbial growth depend on the water source and on the other factors.
Biological growth in circulating water can foul pipelines, accelerate corrosion, attack wood, decrease heat transfer, plug filters, cause imperfections in paper sheets; decompose sizing mixtures, and cause many other process interferences.
Oxidizing biocides including chlorine gas, hypochlorous acid, bromine and other oxidizing biocides are widely used in recirculating water.
xe2x80x9cChlorine demandxe2x80x9d is defined as the quantity of chlorine that is reduced or otherwise transformed to inert forms of chlorine by substances in the water; and standard methods have been established for measuring it. In this specification and claims xe2x80x9cchlorine demandxe2x80x9d is as measured by procedures outlined in xe2x80x9cStandard Methods for the examination of water and waste water,xe2x80x9d, 16th edition, Methods xc2xa7409, pages 316-319. The methods are based on applying a specific dose of chlorine to the medium and measuring the residual chlorine left after a given contact time. Chlorine-consuming substances include ammonia and amino derivatives; sulfides, cyanides, oxidizable cations, pulp lignins, starch, sugars, oil, water treatment additives like scale and corrosion inhibitors.
Microbial growth in the water and in biofilms contribute to the chlorine demand of the water and to the chlorine demand of the system to be treated. Oxidizing biocides were found to be ineffective in waters containing a high chlorine demand, including heavy slimes. Non-oxidizing biocides are usually recommended for such waters.
Chlorination of water having a high content of ammonia or other ammino-derivatives results in the formation of chloramines. Chloramines are described as poor biocides relative to hypochlorous or hypobromous acid. According to literature, chloramines are slow to react and may be more persistent in water systems (The NALCO water handbook, 1988, PCT/US 89/02730 21.6.1989, Great Lakes Chem. Corp. Wat. Sci. Tech. 20 No 11/12, pp. 385-39, 1988, by M. D. Sobsey et al., National Academy of Science, 1980, Drinking Water and Health, Vol. 2, National Academy Press, Washington, D.C.).
Chloramination of drinking water occurs when chlorine reacts with small amounts of ammonia either present in or added to the water.
Traditional chloramination occurs with the addition of free chlorine to the total amount of water for reaction with small amounts of ammonia present in the water, or added to the water in known amounts. Only one reference describes the use of pre-formed monochloramine for the post-disinfection of drinking water (J. Beck et al., Aqua I, 25-33, 1986). In this work, chloramines were formed by mixing ammonium sulfate and hypochlorite solution at a concentration of 1000 ppm; pH was adjusted to 7.5 before the point of dosage to avoid carbonate precipitation.
Chloramines were used to control aftergrowth and biofouling in the surface seawater reverse osmosis plants (Desalination 74, 51-67 (1989) and European Patent Application No. 90108872.4,11.05.90, for Du Pont de Nemours and Company). This patent claims the use of chloramine to inhibit regrowth following dechlorination in liquid process streams containing chlorine degradable organic material, that when in degraded form provides energy and carbon source that is assimilable by microorganisms. The chloramine for the process was made in situ by adding NH3 gas, NH4OH, NH4Cl or (NH4)2SO4. The sources of chlorine were Cl2 gas, NaOCl, Ca(OCl)2 and electrolytically generated chlorine.
Chloramines formed in situ during chlorination of cooling water containing ammonia, is considered to have no biocidal effect in the treatment of cooling towers, since chloramines are quickly stripped due to their high volatility [G. Holz Warth et al., Water Res. 18(1), 1421-1427 (1984)].
The disinfection of highly turbid waste water using chlorine had improved when ammonia was added to the waste water (in situ), when longer contact times were allowed Atasi Khalil Z. et al.; Proc. Annu. Conf. Am. Water Works Assoc., 1988 (Pt. 2), pp. 1763-1770).
Ammonium bromide was not mentioned as a possible source for chloramines. The usual sources are ammonia, ammonium chloride and ammonium sulfate.
Its is a purpose of the invention to provide a process and compositions for killing microorganisms and inhibiting biofouling in waters, especially in cooling waters and aqueous systems having a high chlorine demand waters, and more especially in cooling waters and aqueous systems having a high chlorine demand.
It is another purpose of the invention to provide such a process and compositions that have a high biocidal effect and a high initial rate of kill in high chlorine demand waters.
It is further purpose of the invention to provide such a process and compositions the biocidal effect and the properties of which are constant and predetermined.
Other purposes and advantages of the invention will become apparent as the description proceeds.
The process according to the invention comprises mixing two components, one of which is an oxidant, preferably an active chlorine donor and still more preferably sodium hypochlorite, and the other, an ammonium salt, preferably chosen among halides, sulfates and nitrates, and adding the biocidal concentrate immediately to the aqueous system to be treated. The frequency, duration and concentration should be determined in each individual case so as to be sufficient to control biofouling.
Preferably, the two ingredients are mixed in a specific order, and specifically the oxidant is added to a solution of the ammonium salt. In a preferred form of the invention, the oxidant is NaOCl and is slowly added to a well-mixed solution of the ammonium salt diluted in the range of 0.01% to 2% equimolar to chlorine, preferably until a final concentration of chlorine in the mixture has reached 0.01-1% as chlorine. Either batch or continuous formation of the biocidal stock solution is effective.
The biocidal mixture was found to be more effective than other oxidizing biocides (such as chlorine or bromine) whenever the demand in the water system exceeds 1.8 ppm Cl2 out of 2.0 ppm Cl2 within 60 minutes. The percentages of ingredients are given as weight percent.
The mole ratio N/Cl is preferably 1:1. An excess of N may be used.
The temperature of the water to which the mixture is added, can be in the region of 10-60xc2x0 C. The temperature of the solution of ammonium salt should be 10-30xc2x0 C. when NAOCl is added. The pH is controlled by the concentration of the NAOCl solution; preferably the pH range should be 8.0-12.5. The active ingredient was effective at pH 7 and at pH 8. Some decrease in efficacy was noted at pH 9.
The frequency and duration of treatment and concentrations of active ingredient needed in order to maintain good control of biofouling should be determined in each individual case. However, good control was achieved at a level of 3 mg/l as chlorine (4.2 kg NH4Br for 1000 m3).
The biocidal mixture is very effective for shock treatment of fouled systems, even in cases where the water demand is low, and enables the effective daily use of oxidizing biocides. A level of 9 mg/l (as chlorine) is sufficient to clean a fouled system.
In preferred forms of the invention, the mixture is formed and fed either batchwise or continuously by any suitable means, such as by a liquid metering pump or by gravity.
The invention comprises the solutions prepared as described above.
The following are non-limitative examples of possible applications of the process:
Recirculating cool water
Brewery pasteurizer
Air washer
Evaporative cooling water
Scrubbers
Pond and lagoon water
Closed water-cooling systems
Food plant disinfection
Bleachingxe2x80x94pulp and paper etc.
The process according to the invention is compatible with other water treatment chemicals, corrosion and scale inhibitors, etc.